DAN LINDSTROM remembers looking at a piece of Nebraska farmland six or seven years ago that cost $3,300 an acre. Raised on a farm, he ran the numbers with his brother who is farming the family land and concluded that it was too expensive. He figured that with a 2 to 3 percent return, it made more sense to put the money into a dividend-paying stock and have his brother lease additional land.

A few weeks ago, Mr. Lindstrom said similar land sold for nearly $11,000 an acre.

This is a common story across the farm belt. In Indiana, William C. Ade, who made a fortune in oil and gas exploration in Asia, said he stopped adding to his 1,000 acres when the price passed $5,000 an acre.

“Right now it’s at auction as high as $12,000 an acre,” he said of land to grow corn and soybeans in northwest Indiana. “A poor plot of land went for $8,000.”

Mr. Lindstrom, who is a financial adviser at UBS Wealth Management in Omaha, and Mr. Ade, a member of the investment club Tiger 21, are among investors who say they believe farmland could be headed for a serious drop in values, if not a full-on crash. Of course, many of them have been thinking that for years.

The traditional view of farmland, from the farmer to the agricultural economist to the investment adviser, is, as Mr. Ade put it, “an inflation-proof bond.” (“No one is going to steal it,” he said. “No one is going to default on it. If inflation goes up, it will still be there.”)

After the financial collapse of 2008, most forms of real estate were shunned. But not farmland. Prices shot up, driven by rising commodity prices from global demand, low interest rates in the United States and high auction prices begetting higher prices.

Now commodity prices have fallen. Corn has gone to about $4.60 a bushel this week from over $8 a bushel last year. Soybeans have fallen to about $12.60 a bushel from over $17. Yet the value of farmland for row crops has continued to rise.

“We’re kind of at an inflection point,” said Brent Gloy, a professor of agricultural economics at Purdue University. “We’ve had five years of spectacular profitability that was somewhat unanticipated. The U.S.D.A. was forecasting much lower than this, so it surprised people.”

Yet there are still reasons to think that there will be buyers for land who will hold on to it for decades to come. A report released by U.S. Trust highlighted the graying of America’s farmers and their need to sell or lease their land as they age.

“Can land go up and down?” asked John Taylor, national farm and ranch executive for U.S. Trust, which manages 900 farms for investors. “Sure. But I’ve never seen land go to zero. And with world demand, there is no vacancy factor on good U.S. farmland.”

All of this raises the issue of whether it is time to sell.

Brian C. Duke, vice president for Northern Trust, said that even with the run-up in prices for commodities, the annual return of farmland remains about 3 percent. Since 2000, the value of land in Illinois, for example, has increased 207 percent. For an investment comparison, The Dow Jones industrial average went up 42 percent in that period. Triple-digit appreciation has a way of luring new investors.

More experienced investors said that appreciation isn’t the goal: to realize it you have to sell the land. “The capital gain is nice, ” said Albert Kirchner, who is known as Bud and owned a manufacturing company. “We don’t look at the capital gain. We look at it from a productivity standpoint.”

Mr. Kirchner owns 6,000 acres of corn and soybean land in Illinois, an 8,000-acre cattle ranch in Montana and 1,200 acres of timberland in Florida. But his benchmark since he started investing in land after World War II has remained a 4 percent annual return.

At that number and using Agriculture Department estimates that the average farmland value in Illinois is $7,800, Mr. Kirchner’s land would be worth $46.8 million, with an annual return of $1,872,000.

But for the person who has bought land recently at auction, getting even 4 percent becomes more challenging. At $12,000 an acre, a 4 percent return is $480. Kevin Casner, who farms 2,400 acres in Carrollton, Mo., with his son Adam, said the rents in his area ranged from $150 to $450 an acre. (Those rents were negotiated before commodity prices started to fall, and will presumably drop.)

A landlord can squeeze a farmer only so far on rent. Mr. Lindstrom said that an acre of his family farm was producing 175 bushels of corn, which equals a little more than $740 at $4.25 a bushel. But a farmer has some pretty hefty fixed costs. He said that, per acre, seed is $100, nitrogen is $100, dry fertilizer is $100, taxes are $50, insecticide is $30, chemicals are $50, and cash rent is $300 on average. That is $680 without factoring in the cost of equipment and labor. There is no room to increase the rent in this example by $180 more an acre.

This balance between commodity prices and land values and rents can change. It is why all land investors need to have a long enough time frame. “If you’re wanting to park money for five years, farmland is really not what you should be investing in,” Mr. Duke said. “You need to have that longer-term approach.”

Many farmers and investors fear the past decade could be too much of a good thing.

“In real terms the gain we’ve seen in farmland values over the last 10 years are greater than those we saw in the 1970s,” said Professor Gloy of Purdue. In the early 1980s, farmland prices crashed when interest rates went up and farmers could not continue to borrow to finance their operations.

The two things that could set off a decline are a further drop in commodity prices and higher interest rates.

Matt Ward, one of the owners of Premier Grain Farms in Walker, Iowa, said that a drop of $2 a bushel in the price of corn translated to a loss of about $400 an acre. If all 15,000 acres he farms were planted evenly, that would translate into $6 million less in revenue than the land produced last year.

Investors who have sharecropping agreements that designate a split in the harvest between farmer and owner will see their return reduced immediately. Those who have cash leases are likely to have to negotiate lower rents when the lease is renewed.

Another option for an investor has been a variable cash lease. The owner would accept a lower cash rent up front and negotiate an additional payment if the price of the commodity or the yield was higher. There is now less chance of upside potential.

As for interest rates, the fear is that they will rise and the value of land will fall. Mr. Lindstrom, the farm owner who works for UBS, said there was no way to finance land at today’s rates of 5 percent and make money. Cash buyers, he said, are at risk of losing their principal.

“Could the land go from $10,000 to $15,000 an acre?” he said. “Sure, but not today and not with corn at $4.25. I’d be surprised if we don’t go from $10,000 to $8,000 or $7,000. Land has tripled in the last five to six years.”

Still, this was the man who passed on buying land at $3,300 an acre because he thought it was overpriced. “We just tried to buy more ground and were willing to go to $7,500 an acre,” Mr. Lindstrom said.